another water pump question

[snowleopard]

been having trouble with the water pumps on my yanmar 3gm30s. the water from the saildrive leg goes through a vetus strainer (above waterline) and the pump often refuses to suck water until i disconnect a hose and suck it through myself (yuk).

at first i thought the cover plate had worn and allowed air past, then i see in the manual there should be 0.2mm clearance between impeller and cover plate. when the pump is full of water the gap is too small for water to leak through but it seems as though air in the pipes can escape past the impeller so it doesn't develop enough suction to lift the water over the strainer. the problem seems much worse since moving to a drying mooring which lets water drain out of the pipe.

i guess leading the inlet water direct to the pump without leading it above waterline would cure the problem but i don't want to lose my expensive strainers.

has anyone hit this problem and if so, what is the solution?

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[PaulAG]

Two possibilities spring to mind; 1) any sign of mud or grit in intake as you are on a drying mooring? If so this could wear away the impeller blades and reduce suction; 2) is it possible that a small leak has developed in the filter unit (or elsewhere), allowing air into the system that the pump cannot move quickly enough to pull water? Just a thought.

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[Oldhand]

Had this problem with a 3GM on launch after winter lay-up with a brand new impeller installed. Replaced the rather worn end plate and problem was solved. It appears any air-leakage around the impeller end faces severely reduces the pumps self-priming capabilities. I have never seen any requirement for end plate clearance and can't understand how the pump will self-prime if there is a clearance such as you mention.

Now you are in a mud berth, you are probably pumping more sediment which will decrease component life.

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[rickwat]

I had similar trouble on a 3GM and wound up getting a new pump - had the old one machined up as a spare. Several things you can check though. If the Vetus strainer is the type with a wingnut in the middle there should be a small seal on the bolt between the body and where the centre moulding of the cover seats. This has to be quite thin - I use a rubber O ring but original is a squashy plastic similar to big seal round rim. If too thick cover won't pull down far enough to seal(Presume rim seal looks OK). As for pump itself it does not take much wear to prevent suction. Take the impeller out and check the face behind it, you could also try a new impeller and reverse the front cover if there's any wear on the inside of it. Hopefully you don't need a new pump - about £210 from memory. My machining etc on old pump cost about £70.

Good luck

Rick

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[andyball]

That sounds similar to our boat, although I expect it to happen, since the raw water pump is a metal impeller type, that doesn't provide much suction- certainly not enough to lift water up & through the strainer which is perhaps 50-60cm above the wl.

The only way it'll prime is to close the seacock, then fill the strainer,replace the lid, open the seacock,start the motor, & away it goes.....just have to remember to close the seacock before cleaning the strainer, even though well above the waterline; or repeat the above each time.

If you've got a seacock between the leg & strainer? it may be worth trying.



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[fishermantwo]

I agree with what has been posted so far. Another thing to consider is the pipe to the pump from the strainer. I have found that if there is a bit extra hose hanging down from the strainer to the pump so that a reasonable amount of water remains in the hose and therefore keeps the pump full on shutdown. The strainer just slightly higher in the system than the pump.

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[Forbsie]

With me it was definitely the seal of the strainer. BarryH discovered this and I was lucky enough to be able to nip across to Norris Marine in Isleworth for a replacement while Barry was waiting to get his arm sewn back on in the hospital round the corner. /forums/images/icons/crazy.gif

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